suddenly sound got out of sync
If you set the project frame rate at a given value like 24fps, then add the audio, the sound will be one fixed duration. Then if you alter the frame rate the sound will stretch to fit, however your animation will remain locked at specific frames.

So if you add a keyframe at frame 100 it will always rain at frame 100 even if you change the frame rate.

For example you might have an animation created at 24 fps, then you might want to change frame rate when you add some video that is 30 fps. Now the audio will stretch to match the new frame rate but the animation that you created will not.